Transparent silicone rubber vulcanizable under ambient conditions

ABSTRACT

A mixture of a fume silica treated with bis(trimethylsilyl)amine, a hydroxyl endblocked phenyl methyl polysiloxane, and alkoxy silicon compound and a curing catalyst cure at ambient conditions to a transparent silicone rubber useful as an encapsulant for electrical components.

United States Patent [72] inventor James V. Hartlage Midland, Mich. [211Appl No. 27,170 [22] Filed Apr. 9, i970 [45] Patented Nov. 30, 1971 [73]Assignee Dow Corning Corporation Midland, Mich.

[541 TRANSPARENT SILICONE RUBBER VULCANIZABLE UNDER AMBIENT CONDITIONS 3Claims, No Drawings [52] U.S.Cl n 260/37 SB [5i] C08g5l/04 [50] n.260/37 SB [56] References Cited UNITED STATES PATENTS 2,843,555 7/l958Berridge 260/37 SB UX 3.243.404 3/l966 Martellock 260/37 SB 3,532,664l0/l970 Smith .0 260/37 SB Primary Examiner- Lewis To Jacobs Auorneys-Robert F. Fleming, Jr., Laurence R. Hobey, Harry D. Dingman and Roger H.Borrousch TRANSPARENT SILICONE RUBBER VULCANIZABLE UNDER AMBIENTCONDITIONS This invention relates to a silicone rubber, transparent ancland encapsulants vulcanizable at ambient conditions prepared from mixinga silicon compound, containing silicon-bonded hydrogen atoms and a vinylcontaining organo-silicon polymer in the presence of a platinum catalystare known in the art for use in the electronic industry. Thesecompositions, however, have the disadvantage that many of the materialsused in the electronic industry inhibit the cure. The composition of thepresent invention overcomes this disadvantage.

A transparent silicone rubber is described by Daudt in U.S. Pat. No.3,036,985 and consists essentially of an organopolysiloxane of at least50 mol. percent dimethylsiloxane units and a filler which is a certaincopolymer composed of RSiO (CH 2SiO, (Cl-l;,),,SiO and SiO units. Thiscomposition can be cured at room temperature by adding apolyalkoxysilicate and a catalyst. Daudt uses the special copolymerfiller, since both modified. and unmodified silica fillers produceopaque silicone rubbers. The present invention is an improvement overthe composition described by Daudt in that no special copolymer isrequired and a conventional fume silica can be used after receiving atreatment to provide a hydrophobic surface.

A transparent silicone potting composition is described by Modic in U.S.Pat. No. 3,457,214 and consists essentially of a silanol terminatedpolydi'organosiloxane in which ll to 13.3 percent of the organic groupsare phenyl and the rest are methyl, a resinous copolymer oftrimethylsiloxane units and Si units and a reinforcing silica filler.Modic found that it was necessary to use a certain resinous copolymer ascrosslinkers in order to use a reinforcing silica in obtaining a curedtransparent product. The present invention has unexpectedly found thatthe resinous copolymer of Modic is not required to provide a transparentsilicone rubber when a fume silica is treated with bis(trimethylsilyl)amine.

lt is an object of the present invention to provide a composition whichvulcanizes at ambient conditions to a transparent silicone rubber. Thisand other objects will become apparent from the following detaileddescription.

This invention relates to a silicone rubber composition vulcanizable atambient conditions consisting essentially of (A) from one to 50 parts byweight of a reinforcing fume silica having a surface area of from I00 to500 square meters per gram, the surface of said silica having beentreated with bis(trimethylsilyl)amine to provide from 3.5 to 7 weightpercent carbon based on the total weight of treated silica, (B) 100parts by weight of a hydroxyl endblocked phenyl methyl polysiloxanehaving from to 45 mol percent phenylsil oxane units selected from thegroup consisting of diphenylsiloxane units and phenylmethylsiloxaneunits and any remaining units being dimethylsiloxane units, said phenylmethyl polysiloxane having a viscosity from l,000 to 100,000 cs. at 25C. inclusive, (C) from 0.5 to 10 parts by weight ofan alkoxy siliconcompound selected from the group consisting of tetra-alkoxysilane andalkoxypolysilicate wherein each alkoxy group has from 1 to 5 inclusivecarbon atoms, and (D) a curing catalyst for the composition. 7

The reinforcing fume silica of (A) can be any of the conventional fumesilicas which are readily available commercially and which have asurface area of from 100 to 500 square meters per gram. These fumesilicas are treated to provide a hydrophobic silica with from 3.5 to 7weight percent carbon based on the weight of the treated silica. Onemethod for treating the fume silica with bis(trimethylsilyl)amine is tomix the fume silica and the bis(trimethylsilyl)amine in l the presenceof a small amount of water. The bis(trimethylsilyl )amine is preferablypresent in an amount in excess of that necessary to provide ahydrophobic silica with from 3.5 to 7 weight percent carbon. Thetemperature during treatment can vary broadly including roomtemperature. The treated reinforcing fume silica canbe present in anamount of from 1 to 50 parts by weight per 100 parts of (B). Preferably,the treated reinforcing fume silica is present in an amount of from 10to 35 parts by weight per l00 parts of(B).

The hydroxyl endblocked-phenyl methyl polysiloxane, (B), is apolydiorganosiloxane containing phenylmethylsiloxane units ordiphenylsiloxane units and dimethylsiloxane units wherein any otherunits are present in only trace amounts such as less than one molpercent. The phenylmethylsiloxane units or diphenylsiloxane units arepresent in an amount of from 15 to 45 mol. percent and the remainingunits are dimethylsiloxane units. Preferably, the phenylmethylsiloxaneunits or the diphenylsiloxane units are present in an amount of from 20to mol percent. The hydroxyl endblocked phenyl methyl polysiloxanes,(B'), have a viscosity of from 1,000 to 100,000 cs. at 25 C.,preferablyfrom 1,500 to 40,000 cs. at 25 C. The hydroxyl endblockedphenyl methyl polysiloxane can be a copolymer, a mixture of copolymers,or a mixture of polydimethylsiloxane and a phenyl containing copolymersuch as hydroxyl endblocked polyphenylmethylsiloxane, as long as the molpercent of phenylmethylsiloxane units or diphenylsiloxane units providethe required amount in the total mixture.

The alkoxy silicon compounds, (C), can be tetra-alkoxysilanes or thepartial hydrolyzates thereof known as alkoxy polysilicates. The alkoxygroups can have from 1 to 5 carbon atoms per group and can beillustrated by methoxy, ethoxy, npropoxy, isopropoxy, n-butoxy,isobutoxy and amyloxy. These alkoxy silicon compounds are well known inthe art and the tetra-alkoxysilanes can be illustrated bytetra-ethoxysilane, tetra-n-propoxysilane, tetra-butoxysilane,tetra-amyloxysilane and the various partial hydrolyzates, such asethylpolysilicate and n-propoxypoly silicate. Preferably the alkoxygroups are ethoxy or propoxy. The amount of the alkoxy silicon compoundcan be from 0.5 to 10 parts be weight per parts by weight of (B),preferably from 2 to 6 parts by weight per l00 parts be weight of(B).

The compositions of the present invention can be cured by any of theconventional catalysts used in curing the alkoxyhydroxyl. The curingcatalyst can be illustrated by the condensation product of an aliphaticaldehyde and an aliphatic primary amine as described in U.S. Pat. No.2,833,742 which is hereby incorporated by reference and by the metallicsalt of an organic monocarboxylic acid where the metal can be lead, tin,zirconium, antimony, iron, cadmium, barium, calcium, titanium, bismuthor manganese such as dibutyltindilaurate, dibutyltindiacetate andstannouse octoate as described by U.S. Pat. No. 2,843,555 and U.S. Pat.No. 2,927,907, which are hereby incorporated by reference. The amount ofcuring catalyst, (C) is the amount conventionally used to cure thecomposition as described in the art, such as 0.01 to l0 weight percent.

The compositions of the present invention vulcanize at ambientconditions when all the ingredients are mixed. The mixture of all theingredients, however, can be stored at low temperatures, such as 20 C.It is preferred, however, to package the ingredients in two containers,such as a mixture of the treated reinforcing fume silica and thehydroxyl endblocked phenyl methyl polysiloxane in one package and amixture of the alkoxy silicon compound and the curing catalyst inanother package. When the compositions are to be used the contents ofthe two packages are mixed and vulcanization takes place at ambientconditions. The mixture can also be heated to accelerate the cure. Thepresent invention does not exclude the use of inert diluents such aspolydimethylsiloxane fluids and organic solvents. Materials whichproduce haze, however, should be avoided.

The compositions of the present invention cure to transparent siliconerubber which finds use in encapsulating electrical components, thusenabling a worker to see a defective part and repairing that partwithout removing all the encapsulant from the surrounding electricalcomponents. Furthermore, the cure of the composition of this inventionis not inhibited by various substrates found in electrical equipmentwhich occurs when platinum catalysts are used.

The following examples are presented for illustrative purposes only andshould not be construed as limiting the present invention which isproperly delineated in the claims.

EXAMPLE 1 Transparent silicone rubbers were obtained when a mixture of100 parts by weight of a hydroxyl endblocked phenyl methyl polysiloxaneas described below, 25 parts by weight of a reinforcing fume silicahaving a surface area of 170 square meters per gram treated withhexamethyldisilazane and a small amount of water providing the silicawith a carbon content from 3.5 to 7 weight percent, 3 parts by weight ofethylpolysilicate and 1 part by weight of dibutyltindilaurate wasexposed to ambient conditions. The phenyl methyl polysiloxanes were A. apolysiloxane having 30 mol percent phenylmethylsiloxane units and 70 molpercent dimethylsiloxane units and having a viscosity of 25,200 cs. at25 C.

B. a polysiloxane having 30 mol. percent phenylmethylsiloxane units and70 mol percent dimethylsiloxane units and having a viscosity of 1950 cs.at 25 C.

C. a polysiloxane having mol percent diphenylsiloxane and 85 mol percentdimethylsiloxane units and having a viscosity of 2480 cs. at 25 C.

D. a polysiloxane having 44 mol percent phenylmethylsiloxane units and56 mol percent dimethylsiloxane units and having a viscosity of 2580 cs.at 25 C.

EXAMPLE 2 A mixture of 100 parts by weight of a hydroxyl endblockedphenyl methyl polysiloxane having 30 mol percent phenylmethylsiloxaneunits and 70 mol percent dimethylsiloxane units and having a viscosityof6150 cs. at 25 C., 15 parts by weight of the reinforcing fume silicadefined in example 1, 3 parts by weight of ethylpolysilicate and 1 partby weight of dibutyltindilaurate was exposed to moisture under ambientconditions and cured to a transparent silicone rubber of excellentclarity.

EXAMPLE 3 For purposes of comparison a mixture of parts by weight of ahydroxyl endblocked phenyl methyl polysiloxane having 30 mol. percentdiphenylsiloxane units and 70 mol. percent dimethylsiloxane units andhaving a viscosity of 8020 cs. at 25 C., 15 parts by weight of areinforcing fume silica having a surface area of square meters per gramtreated as described in example 1 but having about 3 weight percentcarbon, 3 parts by weight ethylpolysilicate and 1 part by weightdibutyltindilaurate was exposed to moisture under ambient conditions andcured to an opaque silicone rubber.

We claim:

1. A silicone rubber composition vulcanizable at ambient conditionsprepared by mixing the ingredients consisting essentially of A. from 1to 50 parts by weight ofa reinforcing fume silica having a surface areaof from 100 to 500 square meters per gram, the surface of said silicahaving been treated with bis(tn'methylsilyl)amine to provide from 3.5 to7 weight percent carbon based on the total weight of treated silica,

B. 100 parts by weight of a hydroxyl endblocked phenyl methylpolysiloxane having from 15 to 15 mol percent phenylmethylsiloxane unitsand any remaining units being dimethylsiloxane units, said phenyl methylpolysiloxane having a viscosity from 1,000 to 100,000 cs. at 25 C.inelusive,

C. from 0.5 to 10 parts by weight of an alkoxy silicon compound selectedfrom the group consisting of tetraalkoxysilane and alkoxypolysilicatewherein each alkoxy group has from 1 to 5 inclusive carbon atoms, and

D. a curing catalyst for the composition.

2. The silicone rubber composition in accordance with claim 1 in whichthe reinforcing fume silica is present in an amount of from 10 to 35parts by weight, the viscosity of the hydroxyl endblocked phenyl methylpolysiloxane is from 1,500 to 40,000 cs. at 25 C. and the alkoxy siliconcompound is present in an amount of from 2 to 6 parts by weight.

3. The composition of claim 1 cured by exposure to moisture to atransparent silicone rubber.

2. The silicone rubber composition in accordance with claim 1 in whichthe reinforcing fume silica is present in an amount of from 10 to 35parts by weight, the viscosity of the hydroxyl endblocked phenyl methylpolysiloxane is from 1, 500 to 40,000 cs. at 25* C. and the alkoxysilicon compound is present in an amount of from two to six parts byweight.
 3. The composition of claim 1 cured by exposure to moisture to atransparent silicone rubber.